Double-layer varnish comprising an aqueous base varnish and clear varnish with improved clear varnish wetting limit

ABSTRACT

The use of linear and/or branched polyethylene oxide homopolymers, polypropylene oxide homopolymers and/or polyethylene/polypropylene oxide copolymers with randomly distributed ethylene oxide and propylene oxide units, which may be partly or completely etherified with C1-C4-alkanols, with an average molecular weight of more than 1000 to 8000, for reducing the clear lacquer wetting limits during two-layered lacquering with aqueous base lacquer/clear lacquer and suitable base lacquers.

The invention provides the preparation of two-layered lacquers of thebase lacquer/clear lacquer type with improved clear lacquer wettinglimits by using certain additives in aqueous base lacquers, and aqueousbase lacquers suitable for this purpose. It also provides a process forpreparing the two-layered lacquers using the aqueous base lacquer.

EP-A-0 281 936 discloses water-based base lacquers which contain sheetsilicate dispersions prepared using polypropylene oxide as a protectivecolloid. The polypropylene oxides may contain ethylene oxide units in arandomly distributed form or in the form of a block copolymer structure.In the case of block copolymer polypropylene oxides, those with anaverage molecular weight of 500 to 8000 are suitable. However,polypropylene oxide homopolymers with an average molecular weight of 400to 1000, very particularly preferably about 900, are preferred.

In particular in the automobile mass lacquering sector, the preparationof decorative two-layered top lacquers is increasingly being achieved bythe application of a colour and/or effect providing base lacquerconsisting of an aqueous base lacquer and a protective clear lacquerlayer. The clear lacquer must be applied in a minimum thickness, whichis predetermined by the clear lacquer wetting limits, in order to ensurethe formation of a closed clear lacquer film over the entire substratesurface. This minimum thickness may even be exceeded in practice, forexample to ensure the formation of a closed clear lacquer film in placeswhich are difficult to reach by the application equipment, such as forexample crimping, edges or indentations in a car body. This leads notonly to an undesirably high consumption of clear lacquer but, dependingon the nature of the clear lacquer, problems often also occur with thesag resistance of the clear lacquer on vertical surfaces if thethickness of the clear lacquer is too great.

EP-A-0 663 428 discloses the use of low molecular weight poly-C4-olefinsas additives in water-based base lacquers in order to lower the wettinglimit of the clear lacquer being applied. Stable incorporation ofpoly-C4-olefins in water-based base lacquers requires intensive mixingof the poly-C4-olefin with the aqueous base lacquer or thepoly-C4-olefin is expediently added at a stage well before addingsubstantial amounts of water when preparing the water-based baselacquer.

The object of the invention is to provide water-based base lacquers forsubsequent overpainting with clear lacquers which enable lowering of theclear lacquer wetting limits and application of the relevant clearlacquer layer in the thinnest possible layer and also increase processreliability when coating with the clear lacquer. The water-based baselacquer should be easy to prepare and be stable.

It was shown that this object can be achieved by using one or morepolyethylene and or polypropylene glycols and/or polyols which areoptionally partly or completely etherified with C1-C4-alkanols and havean average molecular weight of more than 1000 to 8000 as additives inwater-based base lacquers. Therefore, the invention provides the use oflinear and/or branched polyethylene oxide homopolymers, polypropyleneoxide homopolymers and/or polyethylene/polypropylene oxide copolymerswith randomly distributed ethylene oxide and propylene oxide units,wherein the terminal hydroxyl groups in the homopolymers or copolymersmay be non-etherified or partly or completely etherified withC1-C4-alkanols, with an average molecular weight of more than 1000 to8000 for reducing the clear lacquer wetting limits during two-layeredlacquering with an aqueous base lacquer and clear lacquer. The averagemolecular weight mentioned is understood to be the calculated averagemolecular weight.

During use, the linear and/or branched polyethylene oxide homopolymers,polypropylene oxide homopolyrners and/or polyethylene/polypropyleneoxide copolymers are added to the aqueous base lacquer prior to itsapplication, preferably in a proportion of 0.5 to 15 wt. %, with respectto its solids content.

The invention also provides an aqueous base lacquer compositioncontaining one or more binders and pigments and optionally fillers,cross-linking agents, organic solvents and/or conventional lacqueradditives which is characterised in that it also contains one or morelinear and/or branched polyethylene oxide homopolymers, polypropyleneoxide homopolymers and/or polyethylene/polypropylene oxide copolymerswith randomly distributed ethylene oxide and propylene oxide units,wherein the terminal hydroxyl groups in the homopolymers or copolymersmay be non-etherified or partly or completely etherified withC1-C4-alkanols, with an average molecular weight of more than 1000 to8000 in a proportion of 0.5 to 15 wt. %, with respect to its solidscontent.

The modified colour and/or effect providing water-based base lacqueraccording to the invention may be used during the preparation ofmulti-layered lacquers, in particular decorative base lacquer/clearlacquer two-layered lacquers. The waterbased base lacquers contain in anappropriate manner ionic or non-ionic stabilised binder systems. Theseare preferably anionic and/or non-ionically stabilised. Anionicstabilisation is preferably achieved by the presence of at least partlyneutralised carboxyl groups in the binder, whereas non-ionicstabilisation is achieved by the presence of lateral or terminalpolyethylene oxide units in the binder. The waterbased base lacquer mayhave physically drying properties or may be cross-linkable with theformation of covalent bonds. Base lacquers which cross-link with theformation of covalent bonds may be self-cross-linking systems or requirethe use of external cross-linking agents. In the latter case, they maybe single component or multi-component water-based base lacquers.

Water-based base lacquers according to the invention contain one or moreconventional film-forming binders. They may also contain cross-linkingagents, if the binders are not self-cross-linking or self-drying.Neither the binders nor the optionally present cross-linking componentsare subject to any restrictions at all. For example, conventionalpolyester, polyurethane and/or poly(meth)acrylate resins may be used asfilm-forming binders. The choice of an optionally present cross-linkingagent is not critical; it is governed in a manner which is familiar to aperson skilled in the art by the functionality of the binder, that isthe cross-linking agents are chosen so that they have a reactivefunctionality which is complementary to the functionality of the binder.Examples of such complementary functionalities between binder andcross-linking agent are: carboxyl/epoxide, hydroxyl/methylolether and/ormethylol (methylol ether and/or methylol preferred as cross-linkingactive groups of aminoplast resins), hydroxyl/free isocyanate,hydroxyl/blocked isocyanate, (meth)acryloyl/CH-acid groups. Providedthey are compatible with each other, several such complementaryfunctionalities may also be present alongside each other in awater-based base lacquer. The cross-linking agents which are optionallypresent in the water-based base lacquer may be present individually oras a mixture.

Water-based base lacquers according to the invention contain colourand/or effect providing pigments and optionally fillers. Examples ofcolour providing inorganic or organic pigments and fillers are titaniumdioxide, micronised titanium dioxide, iron oxide pigments, carbon black,silicon dioxide, barium sulfate, micronised mica, talcum, kaolin, chalk,sheet silicates, azo pigments, phthalocyanine pigments, quinacridonepigments, pyrrolopyrrol pigments, perylene pigments. Examples of effectproviding pigments are metal pigments, e.g. consisting of aluminium,copper or other metals; interference pigments such as e.g. metaloxide-coated metal pigments, e.g. titanium dioxide-coated aluminium,coated mica such as e.g. titanium dioxide-coated mica, graphite effectpigments, platelet-shaped iron oxide, platelet-shaped copperphthalocyanine pigments.

Effect pigments are generally provided in the form of a commerciallyavailable aqueous or non-aqueous paste, optionally with preferablywater-dilutable organic solvents and additives added thereto, and thenmixed with aqueous binder under shear. Powdered effect pigments mayfirst be processed with preferably water-dilutable organic solvents andadditives to form a paste.

Coloured pigments and/or fillers, for example, may be dispersed in someof the aqueous binder. The dispersion procedure may preferably also beperformed in a special water-dilutable paste resin. Dispersion may takeplace in conventional equipment known to a person skilled in the art.Then final coloured pigment dispersion is completed using the remainderof the aqueous binder or aqueous paste resin.

According to the invention, it is an essential feature that thewater-based base lacquer contains the previously defined polyethyleneoxide homopolymers, polypropylene oxide homopolymers and/orpolyethylene/polypropylene oxide copolymers. These may also be calledpolyethylene glycols, polypropylene glycols, polyethylene/polypropyleneglycols optionally partly or completely etherified with C1-C4-alkanolsor the corresponding polyols, provided in the last case that they arebranched products. In the following, for simplification, the expression“polyethylene and/or polypropylene glycols and/or polyols” is chosen.These are polyethylene oxide homopolymers or polypropylene oxidehomopolymers or polyethylene/polypropylene oxide copolymers withrandomly distributed ethylene oxide and propylene oxide units, whereinthe terminal hydroxyl groups in the homopolymers or copolymers may benon-etherified or partly or completely etherified with C1-C4-alkanols.The average molecular weight of the homopolymers or copolymers ispreferably more than 1000 to 4000 in particular about 2000. They may bepolyethylenepolyols and/or polypropylenepolyols with a branchedstructure, which can be prepared by ring-opening polymerisation ofethylene and/or propylene oxide using trifunctional or higher functionalpolyols as starter molecules such as, for example, trimethylolpropane,glycerine, pentaerythritol. In the context of the invention, linearpolyethylene glycols and/or polypropylene glycols are preferred. Thesecan be prepared by ring-opening polymerisation of ethylene and/orpropylene oxide using water or diols as starter molecules, such aspreferably ethylene glycol or propylene glycol. Homopolymerrepresentatives are preferably used as additives in water-based baselacquers, in particular polypropylene oxide homopolyrners. The terminalhydroxyl groups of polyethylene and/or polypropylene glycols and/orpolyols may be partly or completely etherified with C1-C4-alkanols suchas methanol, ethanol, n-propanol, isopropanol, 1-butanol, 2-butanol,isobutanol; preferably, however, the non-etherified hydroxy-functionalpolyether compounds are preferred.

The polyethylene and/or polypropylene glycols and/or polyols are used ina proportion of 0.5 to 15 wt. %, preferably 1 to 10 wt. %, with respectto the solids content of the water-based base lacquer.

Incorporation of the polyethylene and/or polypropylene glycols and/orpolyols may take place at any stage during preparation of thewater-based base lacquer, for example also as an additive to the finalwater-based base lacquer itself, for example as a subsequent correctionagent. Stable incorporation of the polyethylene and/or polypropyleneglycols and/or polyols can be achieved in a mixing process without theuse of special measures.

Water-based base lacquers according to the invention may contain furtherconventional lacquer additives in the amounts conventionally used inlacquers, for example between 0.1 and 5 wt. %, with respect to theirsolids contents. Examples of such additives are antifoaming agents,wetting agents, bonding substances, catalysts, flow control agents,anticrater agents, light protection agents and thickeners such as forexample, synthetic polymers with ionic and/or associative groups such aspolyvinyl alcohol, poly(meth)acrylamide, poly(meth)acrylic acid,polyvinylpyrrolidone, hydrophobic modified ethoxylated polyurethanes orpolyacrylates, cross-linked or non-cross-linked polymericmicroparticles.

Water-based lacquers according to the invention may contain, forexample, conventional lacquer solvents, for example in amounts ofpreferably less than 20 wt. %, particularly preferably less than 15 wt.%. These may be conventional lacquer solvents and may arise, forexample, from preparation of the binder or be added separately. Examplesof such solvents are monohydric or polyhydric alcohols, e.g. propanol,butanol, hexanol; glycol ethers or esters, e.g. diethylene glycoldi-C1-C6-alkyl ethers, dipropylene glycol di-C1-C6-alkyl ethers,ethoxypropanol, butyl glycol; glycols, e.g. ethylene glycol and/orpropylene glycol and their dimers or trimers, N-alkylpyrrolidones, suchas e.g. N-methylpyrrolidone and also ketones such as methyl ethylketone, acetone, cyclohexanone; aromatic or aliphatic hydrocarbons, e.g.toluene, xylene or linear or branched aliphatic C6-C12 hydrocarbons.

Water-based base lacquers according to the invention have solidscontents of, for example, 10 to 50 wt. %; in the case of water-basedeffect base lacquers this range is preferably 15 to 30 wt. %, forwater-based monotone coloured base lacquers the solids content ispreferably greater, for example 20 to 45 wt. %. The ratio by weight ofpigment to binder in the water-based base lacquer is, for example,between 0.05:1 and 3:1; for effect water-based base lacquers the ratiois preferably, for example, 0.1:1 to 0.6:1 and for monotone colouredwater-based base lacquers the ratio is preferably greater, for example0.1:1 to 2.5:1, each with respect to the weight of solids. Whencalculating the ratio of pigment to binder, the sum of the proportionsby weight of colour-providing pigments, effect pigments and fillers iscompared with the sum of the proportions by weight of solid binder,solid paste resin and solid cross-linking agent in the final water-basedbase lacquer.

Aqueous base lacquers according to the invention are suitable forpreparing multilayered lacquers, in particular colour and/oreffect-providing two-layered lacquers in the motor vehicle sector. Theyare suitable for the initial lacquering and repair lacquering of motorvehicles, but they may also be used in other areas, e.g. plasticsmaterials lacquering, in particular for lacquering motor vehicle parts.

Thus, the invention also provides a process for preparing two-layeredlacquers by applying a water-based base lacquer according to theinvention and a clear lacquer. The water-based base lacquers accordingto the invention may be applied to a wide variety of types ofsubstrates. In general, these are metallic or plastics substrates. Theseare often pre-coated, i.e. plastics substrates may be provided with e.g.a plastics primer, metallic substrates generally have, for example, anelectrodeposited primer and optionally an additional one or more lacquerlayers such as e.g. a primer surfacer layer. Water-based base lacquersaccording to the invention are preferably applied by spraying in a drylayer thickness of 8 to 50 μm; for water-based effect base lacquers thedry layer thickness is preferably for example, 10 to 25 μm, forwater-based monotone coloured base lacquers it is preferably higher, forexample 10 to 40 μm. Application preferably takes place using awet-on-wet process, i.e. after an evaporation phase, e.g. at 20 to 80°C., the water-based base lacquer layers are overpainted with aconventional clear lacquer in a dry layer thickness of preferably 30 to60 μm and dried or cross-linked together with this at temperatures of,for example, 20 to 150° C. The drying conditions for two-layeredlacquers consisting of water-based base lacquer and clear lacquer aregoverned by the clear lacquer system used. For repair purposes, forexample, temperatures of 20 to 80° C. are preferred. For purposes ofmass lacquering temperatures of more than 100° C., for example more than110° C., are preferred. The formation of a bloom-like, milky film on theclear lacquer surface, which is often observed in the prior art, cannotbe observed on multilayered lacquers prepared using water-based baselacquers according to the invention, even in the case of darker shades.

Suitable clear lacquers are basically any known clear lacquers ortransparent pigmented surface coating agents. Either solvent-containingsingle component (1C) or two-component (2C) clear lacquers,water-dilutable 1C or 2C clear lacquers, powder clear lacquers oraqueous powder clear lacquer slurries may be used here. 2C polyurethaneclear lacquers are preferably used to overpaint base lacquer layersprepared from water-based base lacquers according to the invention.

As a result of use according to the invention of polyethylene and/orpolypropylene glycols and/or polyols in colour and/or effect providingwater-based base lacquers known per se from the prior art, it ispossible to lower the wetting limits of the clear lacquer surfacecoating agent applied to the base lacquer prepared therefrom. Anothereffect of use according to the invention of polyethylene and/orpolypropylene glycols and/or polyols in water-based base lacquers is theincrease in their sagging limit which may be of significance, forexample, in the case of water-based base lacquers with those shadeswhich have a low hiding power or in the case of water-based baselacquers with an increased tendency to sag, for example highly pigmentedwater-based base lacquers.

EXAMPLE 1 Preparation of a Monotone Red Multilayered Lacquer, Comparison

a) Preparation of a Water-based Monotone Red Base Lacquer

10 parts of a 40 wt. % aqueous acrylate polyester dispersion, 40 partsof a commercial polyesterurethane dispersion (solids content 42 wt. %)and 8 parts of deionised water are blended under a dissolver. Then 6.5parts of an acid acrylate thickener neutralised withdimethylethanolamine, 2 parts of a wetting agent, 3 parts of butylglycol and 2 parts of N-methylpyrrolidone are stirred in to form ahomogeneous mixture. 24.5 parts of an aqueous red pigment paste (ratioby weight of pigment/binder 2.2:1, solids content 43 wt. %) are admixed.Then the mixture is diluted with 4 parts of deionised water. Awater-based monotone red base lacquer with a sagging limit of 24 μm isobtained.

b) Preparing the Multilayered Lacquer

Car body sheets (30×60 cm) which are pre-coated with commerciallyavailable cathodically depositable electrodeposition lacquer used inautomobile mass lacquering (18 μm dry layer thickness) and commerciallyavailable primer surfacer (35 μm), is electrostatically lacquered withthe water-based monotone red base lacquer from example 1a) in a drylayer thickness of 40 μm. After leaving to evaporate for 5 minutes at80° C., a wedge-shaped clear lacquer layer consisting of a commerciallyavailable two-component PU clear lacquer is overpainted by spraying witha maximum dry layer thickness of 50 μm (wedge in longitudinal directionof the sheet). After leaving to evaporate for 5 minutes at roomtemperature, the coating is stoved for 30 minutes at 130° C. (objecttemperature). The longitudinal direction of the test sheet is in thevertical position during this process. The visually perceptible clearlacquer wetting limit is 23 μm.

EXAMPLE 2 Preparing a Monotone Red Multilayered Lacquer, According tothe Invention

a) Preparing a Water-based Monotone Red Base Lacquer

99 parts of the water-based monotone red base lacquer from example 1a)are blended with 1 part of polypropylene glycol (average molecularweight 2000). The sagging limit of the water-based base lacquer is 27μm.

b) Preparing the Multilayered Lacquer

Example 1b) is repeated using the water-based monotone red base lacquerfrom example 2a). The visually perceptible clear lacquer wetting limitis 13 μm.

What is claimed is:
 1. A process for coating an optionally precoatedsubstrate with a two-layer coating comprising: applying an aqueous baselacquer composition to said substrate by spray application; and applyinga clear lacquer layer over said aqueous base lacquer composition;wherein said aqueous base lacquer composition is comprising at least onepolymer selected from the group consisting of linear polyethylene oxidehomopolymers, branched polyethylene oxide homopolymers, linearpolypropylene oxide homopolymers, branched polypropylene oxidehomopolymers, linear polyethylene/polypropylene oxide copolymers withrandomly distributed ethylene oxide and propylene oxide units, branchedpolyethylene/polypropylene oxide copolymers with randomly distributedethylene oxide and propylene oxide units, wherein said polymers haveterminal hydroxyl groups and optionally are partly or completelyetherified with C1-C4-alkanols; wherein said polymers have an averagemolecular weight from more than 1,000 to 8,000; further wherein saidaqueous base lacquer composition has a solids content comprising 0.5-15%of said polymers; whereby said clear lacquer achieves a reduced wettinglimit.
 2. The process according to claim 1, wherein the aqueous baselacquer composition is further comprising at least one film-formingbinder selected from the group consisting of polyester resins,polyurethane resins, poly(meth)acrylate resins, and mixtures thereof. 3.The process according to claim 1, wherein the aqueous base lacquercomposition is further comprising at least one component selected fromthe group consisting of pigments, fillers, cross-linking agents, organicsolvents, and conventional lacquer additives.
 4. The process accordingto claim 1, wherein the substrate is a motor vehicle or parts thereof.5. The process according to claim 1, wherein the polymers have anaverage molecular weight range from 2,000 to 8,000.
 6. The processaccording to claim 5, wherein the aqueous base lacquer composition isfurther comprising at least one film-forming binder selected from thegroup consisting of polyester resins, polyurethane resins,poly(meth)acrylate resins, and mixtures thereof.
 7. The processaccording to claim 5, wherein the aqueous base lacquer composition isfurther comprising at least one component selected from the groupconsisting of pigments, fillers, cross-linking agents, organic solvents,and conventional lacquer additives.
 8. The process according to claim 5,wherein the substrate is a motor vehicle or part thereof.
 9. An aqueousbase lacquer composition comprising at least one polymer selected fromthe group consisting of linear polyethylene oxide homopolymers, branchedpolyethylene oxide homopolymers, linear polypropylene oxidehomopolymers, branched polypropylene oxide homopolymers, linearpolyethylene/polypropylene oxide copolymers with randomly distributedethylene oxide and propylene oxide units, branchedpolyethylene/polypropylene oxide copolymers with randomly distributedethylene oxide and propylene oxide units, wherein said polymers haveterminal hydroxyl groups and optionally are partly or completelyetherified with C1-C4-alkanols; wherein said polymers have an averagemolecular weight from more than 1,000 to 8,000; further wherein saidaqueous base lacquer composition has a solids content comprising 0.5-15%of said polymers; whereby said aqueous base lacquer composition enablesa clear lacquer having a reduced wetting limit to be applied over saidaqueous base lacquer composition.
 10. The aqueous base lacquercomposition according to claim 9, further comprising at least onefilm-forming binder selected from the group consisting of polyesterresins, polyurethane resins, poly(meth)acrylate resins, and mixturesthereof.
 11. The aqueous base lacquer composition according to claim 9,further comprising at least one component selected from the groupconsisting of pigments, fillers, cross-linking agents, organic solvents,and conventional lacquer additives.
 12. The aqueous lacquer compositionaccording to claim 9, wherein the polymers have an average molecularweight range from 2,000 to 8,000.
 13. The aqueous lacquer compositionaccording to claim 12, further comprising at least one film-formingbinder selected from the group consisting of polyester resins,polyurethane resins, poly(meth)acrylate resins, and mixtures thereof.14. The aqueous lacquer composition according to claim 12, furthercomprising at least one component selected from the group consisting ofpigments, fillers, cross-linking agents, organic solvents, andconventional lacquer additives.
 15. A substrate coated with an aqueousbase lacquer composition according to claim
 9. 16. The substrateaccording to claim 15, wherein said substrate is a motor vehicle or partthereof.
 17. A substrate coated with an aqueous base lacquer compositionaccording to claim
 12. 18. The substrate according to claim 17, whereinsaid substrate is a motor vehicle or part thereof.